Dog-clutch coupling device

ABSTRACT

The invention relates to a device for coupling two shafts intended to rotate in the continuation of one another about an axis. The device comprises a dog clutch allowing the driving shaft to drive the driven shaft, and declutching means allowing the dog clutch to be uncoupled from a clutch-engaged position to a declutched position. The declutching means comprise at least one first channel secured to a driving element of the dog clutch, the first channel having the shape of a portion of a torus about the axis, at least one second channel secured to the driving shaft, the second channel having a helical shape about the axis, and a rolling element intended to roll between the first and in the second channel.

The invention relates to a device for coupling two shafts intended torotate in the continuation of one another. A dog clutch couples the twoshafts. A dog clutch generally comprises teeth or dogs belonging to eachof the two shafts. When the teeth (or dogs) collaborate with oneanother, the two shafts are coupled. A dog-clutch coupling device alsocomprises means for separating the teeth of each shaft in order touncouple them. These means will be known as declutching means in theremainder of the description.

Known declutching means entail stopping the rotation of the two shaftsand applying a significant external force in order to separate theteeth. This is because the teeth are generally held in contact with oneanother by means of a spring and it is therefore necessary to overcomethe force of the spring in order to declutch.

As a result, it is not possible to use a known dog-clutch coupling forcoupling a starter motor with an engine if there is a desire to uncouplethe starter motor when the engine has started.

It is an object of the invention to alleviate these drawbacks byproposing a dog-clutch coupling device for which the declutching meanscan uncouple the shafts even when these are rotating and in which thedeclutching means can uncouple the shafts automatically if the resistivetorque between a driving shaft and a driven shaft reverses.

To this end, the subject of the invention is a device for coupling adriving shaft and a driven shaft which are intended to rotate withrespect to a casing in the continuation of one another substantiallyabout an axis, the device comprising a dog clutch allowing the drivingshaft to drive the driven shaft and declutching means allowing the dogclutch to be uncoupled from a clutch-engaged position to a declutchedposition, characterized in that the declutching means comprise at leastone first channel secured to a driving element of the dog clutch, thefirst channel having the shape of a portion of a torus about the axis,at least one second channel secured to the driving shaft, the secondchannel having a helical shape about the axis, and a rolling elementintended to roll between the first and in the second channel.

The invention also makes it possible considerably to reduce the forcenecessary for declutching. By virtue of the invention, the force neededto separate the teeth of the dog clutch is provided no longer by meansexternal to the device but by the device itself and, more specificallyby the rotational energy of the shafts.

The invention also makes it possible significantly to reduce the mass ofthe declutching means and to improve their compactness.

The invention will be better understood and other advantages will becomeapparent from reading the detailed description of one embodiment of theinvention, given by way of example, the description being illustrated bythe attached drawing in which:

FIG. 1 depicts a coupling device according to the invention, in sectionalong the axis of the two shafts; the upper part of the section depictsthe coupling device in the clutch-engaged position and the lower part ofthe section represents the coupling device in the declutched position;

FIGS. 2 a and 2 b depict the channels and the rolling element indevelopment about the axis of rotation of the shafts;

FIG. 3 depicts one exemplary embodiment of the teeth of the dog clutch;

FIG. 4 depicts, in section at right angles to the axis of rotation ofthe shafts, means for braking the driving element and free wheel means.

FIG. 1 depicts a coupling device in section along a plane containing anaxis 1 about which two shafts 2 and 3 can rotate with respect to acasing 4. The shaft 2 is, for example, that of the rotor of an electricmotor used as a starter motor. A rotary bearing comprising, for example,a rolling bearing assembly 7 allows the shaft 2 to rotate with respectto the casing 4, the rotation being about the axis 1. The shaft 3 makesit possible, for example, to couple the electric motor to a transfer box(not depicted).

A dog clutch 9 allows the shafts 2 and 3 to be coupled and uncoupled.According to the example depicted, the dog clutch 9 comprises a firstseries of teeth 10 secured to a driving element 12 of the dog clutch 9and a second series of teeth 11 secured to the shaft 2. The teeth 10 andthe teeth 11 collaborate with one another in such a way as to allow theshaft 2 to be driven by the shaft 3 when the coupling device is in theclutch-engaged position.

Declutching means allow the teeth 10 and 11 to be uncoupled. Thedeclutching means comprise at least one first channel 13, secured to thedriving element 12, and having the shape of a portion of a torus aboutthe axis 1. The declutching means also comprise at least one secondchannel 16 secured to the shaft 2. The second channel 16 has a helicalshape about the axis 1. The declutching means also comprise a rollingelement 17 intended to roll between the first and the second channels 13and 16. The rolling element 17 is advantageously a spherical ball. Anelastic element such as, for example, a helical spring 18 tends to keepthe teeth 10 and 11 uncoupled. The helical spring 18 bears between thedriving shaft 2 and the driving element 12 of the dog clutch 9. Thedriving element 12 can move in a helix with respect to the shaft 2 aboutthe axis 1. It is possible to provide a component of revolution 14 aboutthe axis 1, free to rotate with respect to the driving element 12. Thehelical spring 18 then bears against the driving element 12 via thecomponent of revolution 14 which, driven by the helical spring 18,constantly turns at the same rotational speed as the shaft 2 about theaxis 1. The rotational freedom of the component of revolution 14 withrespect to the driving element 2 may be afforded by a thrust ballbearing 15.

When the teeth 10 and 11 of the dog clutch 9 are coupled and the shaft 2is driving, that is to say exerting a driving torque on the shaft 3, thedriving element 12 is braked in its rotation about the axis 1 by theteeth 11 secured to the shaft 3. The braking of the driving element 12tends to move the driving element 12 with respect to the shaft 2 in itshelical movement so that the teeth 10 and 11 remain coupled.

By contrast, when the shaft 2 no longer exerts a driving torque on theshaft 3, the spring 18 tends to move the driving element 12 with respectto the shaft 2 in its helical movement in the opposite direction to themovement previously described such that the teeth 10 and 11 uncouple.The uncoupling of the teeth 10 and 11 takes place automatically when theshaft 2 ceases to exert a driving torque on the shaft 3. The automaticuncoupling is useful when, for example, the shaft 2 is that of a startermotor and the shaft 3 is that of an engine that the starter motor is tostart. Initially, the starter motor drives the engine and the dog clutch9 remains coupled. Then, when the engine has started up, the dog clutch9 automatically uncouples itself without external intervention.

Advantageously, the second channel 16 is made in a sleeve 19 attached tothe shaft 2. The sleeve 19 is secured to the shaft 2. A connection interms of rotation about the shaft 1 between the sleeve 19 and the shaft2 is made, for example, using splines 8. The sleeve 19 is prevented fromeffecting any translational movement with respect to the shaft 2, forexample, by means of a screw 5 screwed into the shaft 2 and the head ofwhich presses against the sleeve 19 via a washer 6. The sleeve 19 mayadvantageously act as a thrust face for the interior cage of the rollingbearing assembly 7.

Advantageously, the coupling device comprises three first channels 13distributed uniformly about the axis 1, three second channels 16distributed in the same way as the three first channels 13, and threerolling elements 17.

More specifically, a rolling element 17 can roll between one of thethree first channels 13 and one of the three second channels 16. Thesame is true of the other channels 13 and 16 which all operate, inpairs, with a rolling element 17 between each channel 13 and 16 of thepair. The three pairs of channels 13 and 16 and their associated rollingelement 17 are depicted in FIGS. 2 a and 2 b in development about theaxis 1. FIG. 2 a depicts the declutched position and FIG. 2 b depictsthe clutch-engaged position. FIGS. 2 a and 2 b show the three channels13 made in the continuation of one another and running at right anglesto the axis 1. Also visible is the helical shape of the three channels16. The helix angle of the channels 16 is, for example, 10°.

Advantageously, the teeth of each series have complementing triangularshapes, the shapes are made in such a way that when the driving shaft 2drives the driven shaft 3, a continuation of each contacting surface ofeach series of teeth contains the axis 1.

An example of a tooth shape is clearly visible in FIG. 3. In otherwords, when the driving shaft 2 drives the shaft at 3, each tooth 10exerts a force on a corresponding tooth 11 in a direction at rightangles to the axis 1. Conversely, if the driven shaft tends to drive thedriving shaft, each tooth 11 exerts a force on a corresponding tooth 10in a direction such that a component of this force tends to separate thetwo series of teeth 10 and 11 and therefore tends to declutch. FIG. 3 isa partial development about the axis 1. The direction of rotation of theshafts is indicated by the arrow 20.

When the device is stationary, the teeth 10 and 11 of the dog clutch 9are uncoupled because of the action of the helical spring 18 which keepsthe driving element 12 in the position depicted in the lower part ofFIG. 1, known as the declutched position. The spring stiffness of thehelical spring 18 can be selected such that when the driving shaft 2 isset in motion, the inertia of the driving element 12 is enough for therolling elements 17 to move in the channels 13 and 16 and for thedriving element 12 to leave the declutched position and approach theclutch-engaged position depicted in the upper part of FIG. 1. Even ifthe inertia of the driving element 12 is not enough to reach theclutch-engaged position, all that is required is for the teeth 10 and 11to come into contact with one another to cause the inertia of thedriving element 12 to increase and the driving element 12 therefore toreach the clutch-engaged position.

The shaft 2 may be set in motion even if the driven shaft 3 is notstationary. In this case, when the speed of the driven shaft 3,engagement takes place.

Advantageously, clutch engagement can be improved when the driving shaft2 is set in motion by arranging within the device a brake 21 intended toslow the driving element 12 in its rotation with respect to the drivingshaft 2. In practice, the brake 21 can be placed inside the casing 4 andbrake the rotation of the driving element 12. By braking the drivingelement 12 with respect to the casing 4, the driving element 12 isslowed with respect to the driving shaft 2 when the driving shaft 2 isset in motion. The brake is, for example, of the reluctance type so asto avoid any mechanical friction between the casing 4 and the drivingelement 2. In one embodiment given by way of example and depicted inFIG. 4, the brake 21 comprises a plurality of slots 22 made in amagnetic material belonging to the driving element 12, a plurality ofpermanent magnets 23 secured to the casing 4 and in interaction with themagnetic material.

The slots 22 are situated on an exterior cylindrical part 24 of thedriving element 12, which cylindrical part 24 is of the axis 1. Thecylindrical part 24 is made of the magnetic material. The permanentmagnets 23 are situated in an interior cylindrical part 25 of the casing4. The interior cylindrical part 25 is also of axis 1 so that when thedriving element 12 rotates inside the casing 4, each permanent magnet 23interacts magnetically with a slot 22 then with a solid part 26 of themagnetic material of the driving element 12 alternately. A solid part 26separates two slots 22 and there are as many slots 22 as there are solidparts 26 on the exterior cylindrical part 24. The alternatinginteraction of the permanent magnets 23 with the slots 22 and with thesolid parts 26 generates forces which tend to slow the driving element12 in its rotation about the axis 1.

Advantageously, the coupling device comprises means allowing the drivingelement 12 to rotate in just one direction of rotation about the axis 1.

These means are also known as a free wheel. These means comprise, forexample, at least one roller 27 free to rotate with respect to a cage 28secured to the casing 4, a surface of revolution 29 belonging to thedriving element 2 and the axis of which is coincident with the axis 1 ofrotation of the driving element, a ramp 30 belonging to the casing 4 andinclined with respect to a tangent to the surface of revolution 29 at aregion on the surface of revolution where the roller 27 is liable toroll, and an elastic element 31 opposing the movement of the roller 27along the ramp 30.

In FIG. 4, six rollers 27 have been depicted with their ramp 30 andtheir respective elastic element 31. The surface of revolution 31 iscommon to all the rollers 27.

When the driving element 12 attempts to rotate in the directionillustrated by the arrow 32, the rollers 27 become wedged between theirrespective ramps 30 and the surface of revolution 29 and rotation isimpossible. By contrast, when the driving element 12 attempts to rotatein the opposite direction, that is to say the direction illustrated bythe arrow 33, the rollers 27 are able to roll freely between the surfaceof revolution 29 and their respective ramps. In this direction 33, thedriving element 12 is able to rotate.

In addition, when the surface of revolution 29 is near the slots 22 andthe solid parts 26, so that there can be magnetic interaction betweenthe rollers 27 and the surface of revolution, rotation of the drivingelement 12 tends to move the rollers 27 away from the surface ofrevolution 29, thereby compressing the elastic element 31. This makes itpossible to avoid any friction in the rotation of the driving element 12with respect to the casing 4.

1. Device for coupling a driving shaft and a driven shaft which areintended to rotate with respect to a casing in the continuation of oneanother substantially about an axis, the device comprising: a dog clutchallowing the driving shaft to drive the driven shaft; and declutchingmeans allowing the dog clutch to be uncoupled from a clutch-engagedposition to a declutched position, wherein the declutching meanscomprise at least one first channel secured to a driving element of thedog clutch, the first channel having the shape of a portion of a torusabout the axis, at least one second channel secured to the drivingshaft, the second channel having a helical shape about the axis, and arolling element intended to roll between the first and in the secondchannel.
 2. The coupling device as claimed in claim 1, wherein therolling element is a spherical ball and in that a cross section of eachchannel is a portion of a circle of radius more or less equal to that ofthe spherical ball.
 3. The coupling device as claimed in claim 1,wherein it comprises three first channels distributed uniformly aboutthe axis, three second channels distributed in the same way as the threefirst channels, and three rolling elements.
 4. The coupling device asclaimed in claim 1, wherein the dog clutch comprises a first series ofteeth secured to the driving shaft and a second series of teeth securedto the driven shaft, the teeth in each series have complementingtriangular shapes, the shapes are produced in such a way that when thedriving shaft drives the driven shaft a continuation of each contactingsurface of each series of teeth contains the axis.
 5. The device asclaimed in claim 1, wherein it comprises a brake intended to slow thedriving element in its rotation with respect to the driving shaft. 6.The device as claimed in claim 5, wherein the brake is of the reluctancetype.
 7. The device as claimed in claim 6, wherein the brake comprises aplurality of slots made in a magnetic material belonging to the drivingelement, a plurality of permanent magnets secured to the casing and ininteraction with the magnetic material.
 8. The device as claimed inclaim 1, wherein it comprises means allowing the driving element torotate in just one direction of rotation about the axis.
 9. The deviceas claimed in claim 7 comprises means allowing the driving element torotate in just one direction of rotation about the axis, wherein themeans that allow the driving element to rotate in just one direction ofrotation about the axis comprise at least one roller free to rotate withrespect to a cage secured to the casing, a surface of revolutionbelonging to the driving shaft and the axis of which is coincident withthe axis of rotation of the driving element, a ramp belonging to thecasing and inclined with respect to a tangent in the surface ofrevolution at a region on the surface of revolution where the roller isliable to roll, and an elastic element opposing the movement of theroller along the ramp.
 10. The device as claimed in claim 9, wherein thesurface of revolution is near the slots and the solid parts so thatthere can be magnetic interaction between the rollers and the surface ofrevolution.
 11. The coupling device as claimed in claim 2, wherein itcomprises three first channels distributed uniformly about the axis,three second channels distributed in the same way as the three firstchannels, and three rolling elements.
 12. The coupling device as claimedin claim 2, wherein the dog clutch comprises a first series of teethsecured to the driving shaft and a second series of teeth secured to thedriven shaft, the teeth in each series have complementing triangularshapes, the shapes are produced in such a way that when the drivingshaft drives the driven shaft a continuation of each contacting surfaceof each series of teeth contains the axis.
 13. The coupling device asclaimed in claim 3, wherein the dog clutch comprises a first series ofteeth secured to the driving shaft and a second series of teeth securedto the driven shaft, the teeth in each series have complementingtriangular shapes, the shapes are produced in such a way that when thedriving shaft drives the driven shaft a continuation of each contactingsurface of each series of teeth contains the axis.
 14. The device asclaimed in claim 2, wherein it comprises a brake intended to slow thedriving element in its rotation with respect to the driving shaft. 15.The device as claimed in claim 3, wherein it comprises a brake intendedto slow the driving element in its rotation with respect to the drivingshaft.
 16. The device as claimed in claim 4, wherein it comprises abrake intended to slow the driving element in its rotation with respectto the driving shaft.
 17. The device as claimed in claim 2, wherein itcomprises means allowing the driving element to rotate in just onedirection of rotation about the axis.
 18. The device as claimed in claim3, wherein it comprises means allowing the driving element to rotate injust one direction of rotation about the axis.
 19. The device as claimedin claim 4, wherein it comprises means allowing the driving element torotate in just one direction of rotation about the axis. 20 The deviceas claimed in claim 5, wherein it comprises means allowing the drivingelement to rotate in just one direction of rotation about the axis.